Photodisintegration of Ultrahigh-Energy Cosmic Rays by the Universal Radiation Field

We have carried out a detailed calculation to determine the effectiveness of the universal radiation field in disintegrating ultrahigh-energy cosmic-ray nuclei. We conclude that this process cannot lead to a pure-proton interpretation of the air-shower observations above 10⁸ GeV. Photodisintegration by the universal radiation is unimportant on time scales of 10¹⁰ yr or less for helium nuclei below 10¹⁰ GeV and for iron nuclei below 10¹¹-GeV total energy. Photodisintegration by dilute starlight photons in intergalactic space is unimportant on time scales of 10¹⁰ yr or less at all energies.